doc: iio: ad4695: update for calibration support

Calibration support has been added to the ad4695 driver, so update the
documentation to reflect this.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240820-ad4695-gain-offset-v1-3-c8f6e3b47551@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>